Summary
Highlights
Photosynthesis involves using light energy to build carbohydrates. The net equation shows six water molecules and six carbon dioxide molecules, with light, producing glucose (C6H12O6) and oxygen gas. Water enters through the roots, and carbon dioxide enters the leaves via stomata, which is also where oxygen leaves the plant.
The chloroplast is the organelle where photosynthesis occurs, contrasting with mitochondria for cellular respiration. Photosynthesis converts CO2 and water into glucose and oxygen, while cellular respiration does the opposite. Chlorophyll, found in thylakoids, absorbs blue and red light but reflects green light, making plants appear green. A stack of thylakoids is a granum (plural grana), the fluid inside a thylakoid is the lumen, and the fluid inside the chloroplast is the stroma.
Photosynthesis has two stages: light-dependent reactions (in thylakoids) and light-independent reactions (Calvin Cycle, in stroma). Light-independent reactions can occur without light. Light-dependent reactions oxidize water into oxygen and reduce NADP+ to NADPH, also producing ATP via chemiosmosis using ATP synthase. The products are O2, ATP, and NADPH. The Calvin Cycle takes CO2 and reduces it to sugars like glucose, while oxidizing NADPH back to NADP+ and consuming ATP.
In the thylakoid membrane, light strikes photosystem II (P680), exciting chlorophyll electrons, which then move to plastoquinone. Chlorophyll replenishes lost electrons by oxidizing water into oxygen gas, releasing hydrogen ions and electrons. These electrons pass through the cytochrome b6f complex, pumping protons from the stroma into the thylakoid lumen, creating a proton gradient. Electrons then go to plastocyanin and photosystem I (P700), where another photon re-excites them. They then move to ferredoxin and NADP reductase, which uses the electrons and hydrogen ions to reduce NADP+ to NADPH. The proton gradient drives hydrogen ions through ATP synthase, producing ATP.
The Calvin Cycle has three parts: carbon fixation, reduction, and regeneration of RuBP. Carbon dioxide enters the cycle, reacting with ribulose-1,5-bisphosphate (RuBP), catalyzed by the enzyme RuBisCO, to form 3-phosphoglycerate (PGA). Three molecules of 5-carbon RuBP combine with three CO2 molecules (total 15 + 3 = 18 carbons) to form six 3-carbon PGA molecules.
Six ATP molecules phosphorylate PGA into 1,3-bisphosphoglycerate. Then, six NADPH molecules are used to reduce 1,3-bisphosphoglycerate into glyceraldehyde-3-phosphate (G3P). One of the six G3P molecules is used to produce sugars like glucose, while the other five G3P molecules are used, along with 3 more ATP, to regenerate the three RuBP molecules to continue the cycle. The net result of the Calvin cycle is the conversion of three CO2 molecules into one G3P molecule.
To produce one molecule of glucose, two molecules of G3P are needed. This requires doubling the inputs for a single G3P: six CO2 molecules, 18 ATP molecules, and 12 NADPH molecules for the Calvin Cycle.